According to the official website of Media Factory, light novel series Kono Naka ni Hitori, Imouto ga Iru! (この中に1人、妹がいる！, lit. One of Them Is My Younger Sister!) was green-lit for Anime adaptation. Written by Hajime Taguchi (田口 一) and illustrated by CUTEG, the series revolves around the protagonist Shougo who was asked to search for a female mate at school in order to inherit the family business. But instead of looking for a mate, he learned that his younger sister is among one of the girls. Sounds like something super Harem and super unhealthy with a plot of wealthy male turning a school into his Harem that common Galgame would love to offer. However, with a media of light novel, the series could possibly stand out from the crowed.
Summary: The story centers around Shougo Mikadono, the son of an industrial magnate named Kumagorou Mikadono who passed away. As a final wish, Kumagorou gave Shougo a condition before Shougo can inherit the family legacy — find a female mate at school. Shougo transfers into a school to fulfill that condition. However, he learns that he has a younger sister who was separated at birth. He does not know what his long-lost sister looks like, but she happens to be one of the students at his new school. (via ANN)